1. Field of the Invention
The present invention relates to an apparatus for converting radiation image information carried by a storage layer into a television signal sequence, and in particular to such an apparatus usable in an X-ray diagnostics installation.
2. Description of the Prior Art
A device for converting image information contained in a storage layer into an electrical signal sequence, with the storage layer being disposed on the surface of a rotating drum is described in U.S. Letters Pat. No. 3,859,527. A scanning light radiator directed at the storage layer is provided, and the scanning beam and the drum are rotatable relative to each other and axially displaceable for conducting a line scan as well as a change of line. The scan surface is optically connected to a photo-electric transducer.
When recording radiation images, for example x-ray images, layers wherein the image information is stored as electrical charge distribution at a photo semiconductor layer or as excited locations of a phosphorescent material are, as known, employed in addition to photographically effective layers. A television signal sequence is then obtained therefrom by scanning the layer provided with the image information line-by-line by a focussed light beam. The former method is described in, for instance, "Journal of Applied Photographic Engineering 4:178-182 (1978)"; the second is disclosed in U.S. Letters Pat. No. 4,346,295 and in the afore-mentioned U.S. Letters Pat. No. 3,859,527. A description in the context of x-ray images is found on pages 833 through 838 of the periodical "Radiology" September 1983.
When, for instance, recording x-ray images in layers which contain a storage luminophore as the recording means, i.e. a phosphorescent material, a part of the light generated by the x-rays is stored. Given a format of, for example, 400.times.400 mm as is usually employed in x-ray diagnostics of the lung, the exposure plate proceeds into a read-out device after it has been radiated with x-rays. The exposure plate is scanned therein line-by-line with, for instance, a helium-neon laser beam (630 nm). Given employment of the barium fluorochloride bromide activated with europium (Eu) (BaF(ClBr):Eu) as the recording means, this leads to the emission of light centered at a wavelength of 390 nm. The light generated during this read-out is then supplied via a light-conducting system to a photomultiplier in which the television signal sequence is generated.
A problem in devices of the known type is that the probability for stimulation of the stored light by the laser beam is very slight. A reasonable read-out time for practical applications is possible when only a small part of the stored light is read-out because the laser beam then has to be situated on a pixel only for a short time. Enough light must nonetheless be read-out so that the number of electrons triggered in the photocathode of the photomultiplier is sufficiently high, for example, 10 electrons per absorbed x-ray quantum. The statistical fluctuations thus remain slight and have no harmful influence on the primary noise of the absorbed x-ray quanta. If a greater part of the stored light were read-out, then the majority of the stored light will be read-out in the center of the laser beam focus; the intensity of this stimulated light therefore decreases in this region, but far less in the further environment (scattered laser radiation). This would be harmful because the 3-dimensional resolution would be deteriorated due to this "bleeding".
Thus, only a small part of the stored light can be read-out and the intensity of the stimulated light decreases only slightly during the time in which the laser is situated on a pixel. The result is that the total quantity of light stimulated per pixel is approximately proportional to the read-out time. The required read-out time, in turn, is inversely proportional to the percentile light transfer from the storage luminescent layer to the photomultiplier needed to generate an adequate number of photoelectrons in the photocathode (considered as a step-by-step read-out, continuous in real time). Minimum read-out time thus requires maximum light transfer by the light-guiding system.
In a known apparatus, the planar surface of an exposure plate which is covered with a storage luminophore and contains an x-ray exposure is scanned with a laser beam. The stimulated light emerging from the scanned lines is then transferred by a lightguide to a photomultiplier. This transfer of the light from a scan line which, as already indicated above, should usually be about 400 mm long in x-ray exposures, onto a photomultiplier, whose input amounts to a maximum of 175 mm in a commercially available unit, is achieved by flexible lightguides and the like. Ultraviolet lightguides are preferable for the spectrum of the luminophore because the spectrum of the luminophore (spectrum of the stimulated light) extends down to 350 nm. A light transfer of a maximum of 5% of the light triggered in the luminophore onto the multiplier can then be estimated from the data for known lightguides of this type. The low percentage is attributed to the fact that such lightguides are composed of clad silica glass threads. Silica glass, however, has a low refractive index and the difference between the refractive index of the core of the conductor and that of its cladding is therefore slight. Such a conductor can therefore transmit light having an angle of incidence on the surface of the optical fibers which departs only slightly from the perpendicular (low numerical aperture).
A solid lightguide without cladding is employed according to U.S. Letters Pat. No. 4,346,295. Even so, higher light transfer can not be achieved because the guide is composed of synthetic material, for example, acrylic glass. Due to the lack of a cladding (i.e., a direct boundary with air), there is a high difference in refractive index, i.e., the numerical aperture is high. The spectral transmission, however, is low because of the great length required. A similar lightguide of silica glass would be extremely costly. The great length is needed because, given the light transmission from the 400 mm long line onto a photomultiplier having a significantly smaller diameter, a gradual transformation of cross-section is required (referred to as adiabatic light transmission).
A different design is disclosed in U.S. Letters Pat. No. 3,859,527 according to which the exposure plate is clamped to a rotating drum. Scanning is then undertaken by rotating the drum and directing a radial, axially advancing light beam thereon. A substantially complete light transfer from the point of the light emission to the photomultiplier is then possible because the focus of the laser beam on the storage luminophore layer is at rest relative to the photomultiplier and a short lightguide composed, for instance, of silica glass can therefore be employed. A disadvantage of this device is that the maximum rotational speed of the drum is limited due to the imbalance of clamped foils having a luminophore coating which is always somewhat different, which in turn limits the speed of the read-out. Even with respective balancing, the foils would be torn off by the centrifugal force given excessively high rotational speeds.